There are known techniques for joining two metal materials together by a friction stir welding method, as disclosed in e.g., patent document 1 below. FIG. 9 shows a friction stir welding method disclosed in patent document 1.
As shown in FIG. 9, with an aluminum plate 102 placed on an aluminum plate 101, a welding probe 104 of a friction stir welding tool 103 is moved in a Y direction while rotating counterclockwise (in an X direction) so as to join the aluminum plate 101 and the aluminum plate 102 together.
The friction stir welding method described above is known to be applicable to joining materials of different types. The present inventors have attempted to join the aluminum plate 102 and a carbon steel plate, provided in place of the aluminum plate 101. In so doing, the aluminum plate 102 is placed on the carbon steel plate and then the welding probe 104 of the friction stir welding tool 103 is moved in the Y direction while rotating counterclockwise (in the X direction).
The inventors have conducted a tensile test on the plates joined together. A test result showed that a weld zone between the plates was broken under a tensile load lower than a target load. This means that the technique disclosed in patent document 1 is not suitable for joining the materials of different types together, and hence there is a need for a technique suitable for joining the materials of different types.